Start the thread right behind the bead. I do enough turns to lock the thread in and cut the tag end off.
You can't really see this step from the material hiding the hook. I've already cut two pieces to the desired length. I like to have them longer because it's just easier that way. You don't have to have them this long. They could be half this length if you're trying to save on material. When tying the two pieces of yarn in keep in mind that they need to be tied in closer to the bead. I would say tie it 1/4 of an inch back from the bead. Tie in the two pieces by making one loose turn all the way around the shank then make the next turn tighter locking in the wraps. Do a few more turns with a lot of tension. Pull the two pieces in the front back and tie in front of them. I make enough wraps to lock the bead in. Now you can whip finish the fly and are now ready to make the cuts.
Note: When you make the cuts you want to have the two front pieces pointed out so you can get the best results. Your going to cut these flush (close to the hook).
If you have a rotary vise this step is easier when you make the side cuts. If you don't it's not a big deal. You can see in the picture what your trying to do. Make sure you don't cut the material in the back. When cutting the two front pieces, pull tight on each strand with one hand and cut with the other. This way you can see what you're cutting. I like to cut with a straight pair of scissors and clean it up with a short curve pair. You want to cut close because it will make a tight (ball) look. The key is not to see the thread warps so if that happens you made to many wraps when tying the material in.
When you cut the back it will also be an even cut with the back of the scissors. To get the right proportian you will need to make the cut by meassuring it to the alittle past the bend of the hook.